media: dvb-frontends/mxl5xx: add SPDX license identifier

As it is clear that the driver is licensed under the terms of GPLv2-only
by now, add a matching SPDX license identifier to all driver files.

Cc: Ralph Metzler <rjkm@metzlerbros.de>
Cc: Manfred Voelkel <mvoelkel@DigitalDevices.de>
Signed-off-by: NDaniel Scheller <d.scheller@gmx.net>
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/dvb-frontends/mxl5xx.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Driver for the MaxLinear MxL5xx family of tuners/demods
  *

drivers/media/dvb-frontends/mxl5xx.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Driver for the MaxLinear MxL5xx family of tuners/demods
  *

drivers/media/dvb-frontends/mxl5xx_defs.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Defines for the Maxlinear MX58x family of tuners/demods
  *

drivers/media/dvb-frontends/mxl5xx_regs.h

+/* SPDX-License-Identifier: GPL-2.0 */
 /*
  * Copyright (c) 2011-2013 MaxLinear, Inc. All rights reserved
  *